libxml2 is a widely used library for parsing XML formats. Numerous programming languages and applications rely on libxml2 to handle XML processing. In this post, we will discuss an important security vulnerability tracked by CVE-2024-40896, which allows attackers to bypass custom SAX handlers in libxml2 and perform classic XXE (XML External Entity) attacks.

Brief Background

XML parsing is an important function that allows structured data to be represented in applications. XML External Entity (XXE) vulnerabilities occur when XML input is processed containing a reference to an external entity, which could lead to unauthorized file access, server-side request forgery (SSRF), or denial of service (DoS) attacks.

SAX (Simple API for XML) is an event-driven XML parsing API used by libxml2. SAX is designed to be fast and efficient, making it a popular choice for XML parsing. Developers can create custom SAX handlers to handle XML events, such as the start of an element or the end of an element.

The Vulnerability (CVE-2024-40896)

In libxml2 versions mentioned above, the SAX parser can produce events for external entities even if custom SAX handlers try to override the entity content by setting the "checked" property. This allows an attacker to craft malicious XML documents that can bypass the custom SAX handler and execute an XXE attack on the target application.

Here's a code snippet showing the vulnerability in action

#include <stdio.h>
#include <libxml/parser.h>

static xmlEntityPtr
custom_get_entity(void *userData, const xmlChar *name) {
    xmlEntityPtr entity = xmlGetPredefinedEntity(name);

    if (entity == NULL) {
        // Custom SAX handler should override external entity content
        entity = xmlNewEntity(NULL, name, XML_INTERNAL_GENERAL_ENTITY,
                              BAD_CAST "<!-- ENTITY NOT ALLOWED -->",
                              NULL, NULL);

        // Mark as a "checked" entity to prevent further processing
        entity->checked = 1;
    }
    return entity;
}

int main(int argc, char **argv) {
    xmlDocPtr doc;
    xmlSAXHandler saxHandler = {};

    // Set our custom entity handler
    saxHandler.getEntity = custom_get_entity;

    // Parse an XML document with a custom SAX handler
    doc = xmlReadFile("malicious.xml", NULL, XML_PARSE_NOENT | XML_PARSE_DTDLOAD);
    if (doc != NULL) {
        printf("Parsed document successfully\n");
        xmlFreeDoc(doc);
    } else {
        printf("Error parsing document\n");
    }
    return ;
}

In this code snippet, the custom_get_entity function should override the content of external entities with a predefined comment string, and then set the "checked" property to prevent further processing. However, due to the vulnerability, the SAX parser still produces events for the external entities, allowing an attacker to bypass the custom handler and perform an XXE attack.

Exploit details

An attacker would exploit this vulnerability by crafting a malicious XML file containing external entities. When parsed by an affected version of libxml2, the custom SAX handler will not be able to properly prevent the external entity processing, thus allowing XXE attacks.

References

- Original libxml2 issue and fix: https://gitlab.gnome.org/GNOME/libxml2/-/issues/149
- This link provides more details on XXE attacks: https://owasp.org/www-project-top-ten/2017/A4_2017-XML_External_Entities_(XXE)
- The CVE entry for this vulnerability can be found here: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2024-40896

Conclusion

CVE-2024-40896 is a critical vulnerability affecting libxml2. Users of affected versions should update to a patched version as soon as possible to prevent XXE attacks. Staying up-to-date with security updates is essential for safeguarding your applications from potential exploits.

Timeline

Published on: 12/23/2024 17:15:08 UTC
Last modified on: 12/24/2024 03:15:06 UTC